1. Field of the Invention
The present invention relates to hearing aids. More specifically, the invention relates to a method of processing a signal in a hearing aid. The invention further relates to a hearing aid implementing said method of processing a signal.
Basically, a hearing aid picks up an input signal and sends out a processed output signal. Said processing involves amplification of said input signal according to the user's needs. Amplification is carried out in an amplifier, usually including a compressor having a compressor gain.
Generally, the hearing loss of a hearing impaired is not linear. That is to say, the hearing ability may be almost normal at some sound pressure levels—typically at louder sound pressure levels, while being quite poor at other sound pressure levels—typically at softer sound pressure levels. The fact that amplification is needed especially for the softer sound pressure levels while not so much for the louder sound pressure levels is quite typical for many of the hearing impaired.
2. The Prior Art
State of the art hearing aids are adapted to compensate for this common pattern of hearing loss, by means of a compressor. The compressor is adapted for adjusting the gain so as to vary with the current sound pressure level of the input signal. The variation of the level dependent compressor gain is defined in a compression characteristic. A state of the art hearing aid may include a compression characteristic for each frequency band of the input signal.
Examples of hearing aids wherein the input signal is amplified in a compressor having a compressor gain that varies with sound pressure level in accordance with a compression characteristic are described in EP-B1-1059016 and EP-B1-0824845.
Traditionally, fitting of the hearing aid includes adjusting the compressor gain according to a general compression characteristic, in the following referred to as the standard rationale. The standard rationale takes into account the individual hearing loss, but is apart from that intended to accommodate the average hearing aid user.
However, even though the hearing loss of many hearing impaired follow the above-described pattern regarding the need for a larger amplification of softer sound pressure levels but not necessarily an equally large amplification of louder sound pressure levels, individual differences exist. The need for amplification for one hearing impaired may even vary greatly from that of another having a similar hearing loss.
In an effort to make a conventional hearing aid compensate better for the specific hearing loss and preferences of the individual user, the hearing aid is furthermore fine-fitted to the individual user. The fine-fitting is traditionally carried out as additional adjustments to the standard fitting according to the standard rationale.
One of the problems with the existing way of fine-fitting a hearing aid to the individual user is that the compressor only provides limited possibility for fine adjusting the compression characteristic so as to fit the hearing loss of the individual user sufficiently accurate. This is due to the fact that the number of adjustment points, in each of which the compression characteristic of the compressor can be adjusted independently of the other adjustment points, is traditionally very limited. In many cases, the compression characteristic only has two adjustment points. Hence, adjustment of the compressor gain for one sound pressure level influences that of many other sound pressure levels, which may not be desirable.
Hence, only a crude fitting of the compression characteristic to the hearing loss of the individual user is possible. This means that when fitting a hearing aid to an individual user, a compromise must be made between on one hand providing a sufficient amplification of the input signal for some sound input levels while on the other hand avoiding to amplify the input signal for other sound pressure levels to such an extent that the comfort level of the user is exceeded.
Another problem concerning the existing way of processing the input signal of a hearing aid is associated with signal optimisation. Implementation of various types of adaptive processing such as for instance speech intelligibility optimisation are becoming more widespread in the signal processing in hearing aids.
Unfortunately, fine-adjustments carried out on the compression characteristic during the fine-fitting of the hearing aid to the individual hearing aid user may be regarded as deviations from the optimal compression characteristic and may therefore to a great extent be eliminated or reduced by the adaptive processing. Hence, the effect of the fine-fitting of the hearing aid to the individual user is to a great extent never experienced by the user.
All in all, the existing method of processing and fitting has difficulties in meeting any requirement of individual deviation from the above described typical pattern of hearing loss of many hearing impaired.
Hence, a need for a more flexibly adjustable compressor gain exists so as to be able to fit the actual hearing loss and individual preferences of the hearing aid user better.
Also, there is a need for a manner of avoiding that the effect of any fine-fitting is reduced or eliminated by other processing in the hearing aid, such as for instance adaptive processing.